The properties of nematic or nematic-cholesteric liquid crystalline materials are increasingly utilized for electro-optical display elements wherein significant changes are effected in the optical properties of such materials, such as light absorption, light scattering, birefringence, reflectance or color, under the influence of electric fields. The functioning of display elements of this type is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases (DAP effect), the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the technical application of these effects in electronic components, liquid crystalline dielectrics are required which must fulfill a large number of requirements. Chemical resistance to moisture, air and physical influences, such as heat, and electric fields is of particular importance. Industrially usable liquid crystalline dielectrics are also required to have a liquid crystalline mesophase in the temperature range from at least +10.degree. C. to +50.degree. C., preferably from 0.degree. to 60.degree. C., and the lowest possible viscosity at room temperature, which preferably should not exceed 70.times.10.sup.-3 Pa.s. Finally, they must not have any characteristic absorption in the visible spectrum, i.e., they must be colorless.
A number of liquid crystalline compounds has already been disclosed. These fulfill the stability demands made for dielectrics intended for electronic components, and are also colorless. They include, in particular, the p,p'-disubstituted phenyl benzoates described in German Offenlegungsschrift No. 2,139,628 and the p,p'-disubstituted phenylcyclohexane derivatives described in German Offenlegungsschrift No. 2,636,684. In the two classes of compounds mentioned, and also in other known series of compounds having a liquid crystalline mesophase, there are no individual compounds which form a liquid crystalline nematic mesophase in the required temperature range from 10.degree. C. to 60.degree. C. Therefore, as a rule, mixtures of two or more compounds are prepared in order to obtain substances which can be used as liquid crystalline dielectrics.
For this purpose, a compound having a low melting point and clear point is usually mixed with another compound having a markedly higher melting point and clear point. This normally gives a mixture, the melting point of which is below that of the lower-melting component, while the clear point is between the clear points of the components. It is, however, not easy to prepare optimum dielectrics in this way, since the components having the high melting points and clear points frequently also impart a high viscosity to the mixtures. As a result, the switching times of the electro-optical display elements produced with these mixtures are undesirably extended.
Among the known base material for liquid crystalline dielectrics, the mentioned p,p'-disubstituted phenylcyclohexanes are distinguished by a particularly low viscosity. As is known, the cyclohexane ring in compounds of this type can be present either in the cis-configuration or in the trans-configuration; however, liquid crystalline properties have only been found for the compounds having the trans-configuration. In the conventional syntheses of liquid-crystalline compounds having this structure, an isomerization reaction and/or an isomer separation, which is sometimes associated with considerable loss of substances, is therefore generally necessary. As a result, the manufacture of the phenylcyclohexanes becomes expensive.